Fix back cover artwork scaling — use explicit width/height instead of inset

object-fit: cover on <img> requires explicit dimensions; inset:0 alone
does not force the element size in Chromium. Mirrors the same approach
already used for .front-artwork.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/cover.html

@@ -195,7 +195,9 @@
     /* Optional back cover artwork */
     .back-artwork {
       position: absolute;
-      inset: 0;
+      top: 0; left: 0;
+      width: 100%;
+      height: 100%;
       object-fit: cover;
       opacity: 0.35;
     }